Progress in facsimile and related fields has developed to such an extent that signals derived from graphical and printed or typewritten documents may be economically and accurately transmitted from one location to another. Although the advances in these fields have made possible the development of systems having relatively low cost, ease of operation, and reliability, there remains the need for an increase in the speed of transmission required to satisfactorily transmit a facsimile signal.
Most graphical and printed or typewritten documents include a very substantial amount of redundant information, such as the background or "white" color upon which the contrasting or "black" intelligence information appears. Further, such graphical and printed documents exhibit a considerable degree of spatial correlation. The spatial correlation between signals found between signals along a single line path is effectively utilized in the prior art system known as run-length coding wherein a length of black or white run is coded.
In order to utilize the run-length coding technique, the document must be scanned at a speed compatible with the speed at which the image signal is encoded. One example of scanners is a solid-state image scanner which includes an array of photodiodes successively arranged in a row of 2,048 elements and is arranged to be successively driven by clock pulses to scan a line path so that each line path is resolved into 2,048 picture elements and for each line scan a sequence of 2,048 bits of video information is generated from the photodiode array.
Although the facsimile signal may be transmitted at a speed much higher than is transmitted without coding, the speed of image reconstruction at the receiver station will place limitations on the overall transmission time since the transmitter will have to await after transmission of each line video signal until the receiver station becomes ready to accept the next video signal if substantial length of time is involved in the image reconstruction. In the prior art system, a fixed length of time must be allowed for the transmitter to send the next video signal resulting in a loss of substantial period of time for transmission.